Mammal density and patterns of ectoparasite species richness and abundance

Patterns of species richness, prevalence and abundance of ectoparasites have rarely been investigated at both the levels of populations and species of hosts. Here, we investigated the effects in changes in small mammal density on species richness, abundance and prevalence of ectoparasitic fleas. The comparative analyses were conducted for different small mammal species and among several populations during a long-term survey. We tested the hypothesis that an increase in host density should be linked with an increase in parasite species richness both among host species and among populations within host species, as predicted by epidemiological models. We also used host species density data from literature. We found that host density has a major influence on the species richness of ectoparasite communities of small mammals among host populations. We found no relationship between data of host density from the literature and parasite species richness. In contrast with epidemiological hypotheses, we found no relationships between abundance, or prevalence, and host density, either among host species or among host populations. Moreover, a decrease in abundance of fleas in relation with an increase in host density was observed for two mammal species (Apodemus agrarius and A. flavicollis). The decrease or the lack of increase in flea abundance in relation with an increase in host density suggests anti-parasitic behavioural activities such as grooming.     

Patterns of parasite species richness of Western Palaeartic micro-mammals: island effects

We investigate the patterns of parasite species richness of small mammals of the Western Palaearctic, and the effects of insularity. We compiled 28 studies dealing with 16 species of mammals. There was a significant effect of area size on the total parasite species richness, controlling for host sample size. Four mammal species were sufficiently sampled. Two are associated with humans: Mus musculus, and Rattus rattus , and two are not associated with humans, Apodemus sylvaticus and Clethrionomys glaerolus. We show that there is an effect of insularity on parasite species richness of A. sylvaticus . Rodents associated with humans show no effect of insularity on parasite species richness. These rodents appear to reflect a more complex pattern of parasite species richness, probably because of their close association with humans. Host specificity of parasites significantly decreases on islands, with the exception of Rattus rattus . We use parasitological data in order to explore the geographical connections between host populations. We show that parasites may help to resolve these connections but we emphasise that parasites are better geographic than phylogenetic markers.     

Latitudinal gradient in the taxonomic composition of parasite communities

Although latitudinal gradients in diversity have been well studied, latitudinal variation in the taxonomic composition of communities has received less attention. Here, we use a large dataset including 950 surveys of helminth endoparasite communities in 650 species of vertebrate hosts to test for latitudinal changes in the relative contributions of trematodes, cestodes, nematodes and acanthocephalans to parasite assemblages. Although the species richness of helminth communities showed no consistent latitudinal variation, their taxonomic composition varied as a function of both host type and latitude. First, trematodes and acanthocephalans accounted for a higher proportion of species in helminth communities of fish, whereas nematodes achieved a higher proportion of the species in communities of bird and especially mammal hosts. Second, the proportion of trematodes in helminth communities of birds and mammals increased toward higher latitudes. Finally, the proportion of nematodes per community increased toward lower latitudes regardless of the type of host. We present tentative explanations for these patterns, and argue that new insights in parasite community ecology can be gained by searching for latitudinal gradients not only in parasite species richness, but also in the taxonomic composition of parasite assemblages.     

Density, body mass and parasite species richness of terrestrial mammals

We investigated the relationships between helminth species richness and body mass and density of terrestrial mammals. Cross-species analysis and the phylogenetically independent contrast method produced different results. A non-phylogenetic approach (cross-species comparisons) led to the conclusion that parasite richness is linked to host body size. However, an analysis using phylogenetically independent contrasts showed no relationship between host body size and parasite richness. Conversely, a non-phylogenetic approach generated a negative relationship between parasite richness and host density, whereas the independent contrast method showed the opposite trend – that is, parasite richness is positively correlated with host density. From an evolutionary perspective, our results suggest that opportunities for parasite colonization depend more closely on how many hosts are available in a given area than on how large the hosts are. From an epidemiological point of view, our results confirm theoretical models which assume that host density is linked to the opportunity of a parasite to invade a population of hosts. Our findings also suggest that parasitism may be a cost associated with host density. Finally, we provide some support for the non-linear allometry between density and mammal body mass (Silva and Downing, 1995), and explain why host density and host body mass do not relate equally to parasite species richness.     

The Evolution of Taxonomic Diversity in Helminth Assemblages of Mammalian Hosts

Several studies have searched for the key forces behind the diversification of parasite assemblages over evolutionary time. All of these studies have used parasite species richness as their measure of diversity, thus ignoring the relatedness among parasite species and the taxonomic structure of the assemblages. This information is essential, however, if we want to elucidate which processes have caused an assemblage of parasites to acquire new species. Here, we performed a comparative analysis across 110 species of mammalian hosts in which we evaluated the effects of four host traits (body mass, population density, geographic range, and basal metabolic rate) on the diversity of their assemblages of helminth endoparasites. As measures of diversity, we used parasite species richness, as well as the average taxonomic distinctness of the assemblage and its variance; the latter measures are based on the taxonomic distance between two parasite species, computed across all possible species pairs in an assemblage. Unlike parasite species richness, both the average taxonomic distinctness and its variance were unaffected by the number of hosts examined. These two measures of parasite diversity also proved highly repeatable among host populations of the same mammalian species; in contrast, parasite species richness was unreliable as a species character, as it varied as much within a host species than among different host species. Using phylogenetically independent contrasts, and correcting for potential confounding variables, we found that host population density correlated positively with parasite species richness. There were, however, no other relationships between any of the four host traits investigated and either of our measures of parasite diversity. The processes facilitating the taxonomic diversification of parasite assemblages thus remain unclear, but their elucidation will be necessary if we are to fully understand parasite evolution.     

Richness,nestedness, and randomness in parasite infracommunity structure

Within a host population, parasite infracommunities vary in both richness and species composition. If interspecific interactions among parasites are important in shaping infracommunities, the structure of these assemblages is expected to differ from the one predicted by null models, i.e. from the one that would result from chance alone. Using data from the literature, I tested for discrepancies between observed and random patterns in the richness and composition of gastrointestinal helminth infracommunities of birds and mammals. Both the Poisson distribution and a more sophisticated null model, derived from prevalence of the different parasite species in the host population, usually provided a good fit to the observed distributions of infracommunity richness among hosts. This suggests that parasite species do not co-occur more or less frequently than expected by chance. In mammals, the co-occurrence of all available parasite species in the same host individual, or maximum potential infracommunity richness, was less likely to be observed when several parasite species were available; this is also a phenomenon expected from the random assembly of parasite species. Finally, there was no evidence for a nested subset pattern among parasite species in a host population: rate species were distributed independently of common ones. The overall picture emerging from these results is one in which parasite assemblages are more likely to be the product of random events than of predictable and repeatable processes.     

Speciation and diversification of parasite lineages: an analysis of congeneric parasite species in vertebrates

The evolutionary diversification of living organisms is a central research theme in evolutionary ecology, and yet it remains difficult to infer the action of evolutionary processes from patterns in the distribution of rates of diversification among related taxa. Using data from helminth parasite communities in 76 species of birds and 114 species of mammals, the influence of four factors that may either be associated with or modulate rates of parasite speciation were examined in a comparative analysis. Two measures of the relative number of congeneric parasite species per host species were used as indices of parasite diversification, and related to host body mass, host density, latitude, and whether the host is aquatic or terrestrial. The occurrence of congeneric parasites was not distributed randomly with respect to these factors. Aquatic bird species tended to harbour more congeneric parasites than terrestrial birds. Large-bodied mammal species, or those living at low latitudes, harboured more congeneric parasites than small-bodied mammals, or than those from higher latitudes. Host density had no apparent association with either measures of parasite diversification. These patterns, however, reflect only the present-day distribution of parasite diversification among host taxa, and not the evolutionary processes responsible for diversification, because the apparent effects of the factors investigated disappeared once corrections were made for host phylogeny. This indicates that features other than host body size, host density, latitude, and whether the habitat is terrestrial or aquatic, have been the key driving forces in the diversification of parasitic helminth lineages. This revised version was published online in July 2006 with corrections to the Cover Date.     

Spatial variation in species diversity and composition of flea assemblages in small mammalian hosts: geographical distance or faunal similarity?

Aim Spatial variation in the diversity of fleas parasitic on small mammals was examined to answer three questions. (1) Is the diversity of flea assemblages repeatable among populations of the same host species? (2) Does similarity in the composition of flea assemblages among populations of the same host species decay with geographical distance, with decreasing similarity in the composition of local host faunas, or with both? (3) Does the diversity of flea assemblages correlate with climatic variables? Location The study used previously published data on 69 species of small mammals and their fleas from 24 different regions of the Holarctic. Methods The diversity of flea assemblages was measured as both species richness and the average taxonomic distinctness of their component species. Similarity between flea assemblages was measured using both the Jaccard and Morisita–Horn indices, whereas similarity in the composition of host faunas between regions (host ‘faunal’ distance) was quantified using the Jaccard index. Where appropriate, a correction was made for the potentially confounding influence of phylogeny using the independent contrasts method. Results Flea species richness varied less within than among host species, and is thus a repeatable host species character; the same was not true of the taxonomic distinctness of flea assemblages. In almost all host species found in at least five regions, similarity in flea assemblages decreased with increases in either or both geographical and faunal distance. In most host species, the diversity of flea assemblages correlated with one or more climatic variable, in particular mean winter temperature. Main conclusions Spatial variation in flea diversity among populations of the same mammal species is constrained by the fact that it appears to be a species character, but is also driven by local climatic conditions. The results highlight how ecological processes interact with co‐evolutionary history to determine local parasite biodiversity.     

Mammalian metabolism, longevity and parasite species richness

Basal metabolic rate (BMR) scales allometrically with body mass in mammals, but the reasons why some species have higher or lower metabolic rates than predicted from their body mass remain unclear. We tested the idea that parasite species richness may be a contributory factor by performing a comparative analysis on 23 species of mammals for which data were available on parasite species richness, BMR, body mass and two potentially confounding variables, i.e. host density and host longevity. Parasite species richness was positively correlated with BMR and negatively correlated with host longevity independent of body mass.     

Helminth communities of four commercially important fish species from Chetumal Bay, Mexico

The relative importance of ecology and evolution as factors determining species richness and composition of the helminth communities of fish is a matter of current debate. Theoretical studies use host-parasite lists, but these do not include studies on a temporal or spatial scale. Local environmental conditions and host biological characteristics are shown to influence helminth species richness and composition in four fish species (Eugerres plumieri, Hexanematichthys assimilis, Oligoplites saurus, and Scomberomorus maculatus) in Chetumal Bay, Mexico. With the exception of H. assimilis, the helminth communities had not been previously studied and possible associations between environmental and host biological characteristics as factors determining helminth species richness and composition using redundancy analysis (RDA) are described. Thirty-four helminth species are identified, with the highest number of species (19 total (mean = 6.3 +/- 2.1)) and the lowest (9 (4.0 +/- 1.0)) occurring in H. assimilis and S. maculatus, respectively. The larval nematodes Contracaecum sp. and Pseudoterranova sp. were not only the helminth species shared by all four host species but also were the most prevalent and abundant. Statistical associations between helminth community parameters and local ecological variables such as host habitat use, feeding habits, mobility, and time of residence in coastal lagoons are identified. Phylogeny is important because it clearly separates all four host species by their specialist parasites, although specific habitat and feeding habits also significantly influence the differentiation between the four fish species.     

Host population density and body mass as determinants of species richness in parasite communities: comparative analyses of directly transmitted nematodes of mammals

Epidemiological theory predicts positive correlations between host population density or body mass and species richness among parasite communities. Here I test these predictions by a comparative study of communities of directly transmitted mammalian parasites, gastrointestinal strongylid nematodes. I use data from 45 species of mammals, representing examination of 17 200 individual hosts. The variable studied was the average number of gastrointestinal strongylid nematode species per host population, and three different methods were used to obtain estimates of parasite species richness that are unbiased by number of host individuals examined. Analyses were done using the phylogenetically independent contrast method. Host population density and parasite species richness were strongly positively correlated when the effects of host body weight had been controlled for. Controlling for other variables did not change this, and the relationship was found regardless of method used to correct for uneven sampling effort among host species. A positive relationship between parasite species richness and host body weight was also found, but the effect of host densities had to be controlled for to see this. These relationships between host traits and species richness of directly transmitted parasites are stronger than patterns found using data on indirectly transmitted mammalian parasites, and suggests that links between host traits and parasite species richness are stronger than previously suggested. The results are consistent with parasite species richness being positively linked to pathogen transmission rates and reductions in transmission rates possibly increasing extinction probabilities in parasite populations. The results also suggest that parasites may exert a cost of increases in rate of population energy usage, and thus show that pathogens may be important in generating independence between body mass and rate of population energy usage among host species.     

Taxonomic and geographic bias in the genetic study of helminth parasites

The use of genetic information is now fundamental in parasite taxonomy and systematics, for resolving parasite phylogenies, discovering cryptic species, and elucidating patterns of gene flow among parasite populations. The accumulation of available gene sequences per geographical area or per parasite taxonomic group is likely proportional to species richness, but not without some biases. Certain areas and certain taxonomic groups receive more research effort than others, possibly causing a deficit in the relative number of parasite species being characterized genetically in some areas or taxonomic groups. Here, we use data on the number of parasite records per country or helminth family from the London Natural History Museum host-parasite database, and matching data on the number of gene sequences available from the National Center for Biotechnology Information (NCBI) GenBank database, to determine how available gene sequences scale with species richness across countries or parasitic helminth families. Our quantitative analysis identified countries/regions of the world and helminth families that have received the most effort in genetic research. More importantly, it allowed us to generate lists (based on residuals from the statistical model) of the 20 countries/regions and the 20 helminth families with the largest deficit in available gene sequences relative to their helminth species richness. We propose these lists as useful guides toward future allocation of effort to maximise advances in parasite biodiscovery, systematics and population structure.     

The decay of similarity with geographical distance in parasite communities of vertebrate hosts

Aim The rate at which similarity in species composition decays with increasing distance was investigated among communities of parasitic helminths in different populations of the same host species. Rates of distance decay in similarity of parasite communities were compared between populations of fish and mammal hosts, which differ with respect to their vagility and potential to disperse parasite species over large distances. Location Data on helminth communities were compiled for several populations of three mammalian host species (Ondatra zibethicus, Procyon lotor and Canis latrans) and three fish host species (Perca flavescens, Catostomus commersoni and Esox lucius) from continental North America. Methods Distances between localities and similarity in the composition of helminth communities, the latter computed using the Jaccard index, were calculated for all possible pairs of host populations within each host species. Similarity values were then regressed against distance to see if they decayed at exponential rates, as reported for plant communities; the significance of the regressions was assessed using randomization tests. Results The number of hosts examined per population did not correlate with the number of helminth species found per population, and thus sampling effort is unlikely to have confounded the results. In four (two mammals and two fish) of the six host species, similarity in helminth communities decayed exponentially with distance. When the log of similarity is plotted against untransformed distance, the slopes obtained for the two fish species are lower than those obtained for the two mammalian host species. Main conclusions Similarity in the composition of parasite communities appears to decay exponentially with increasing distance in some host species, but not in all host species. The rate of decay is not necessarily associated with the vagility of the host. Although distance decay of similarity is generally occurring, it seems that other ecological processes, related either to the host or its habitat, can obscure it.     

Parasite assemblages of Australian species of Pseudomys (Rodentia: Muridae: Murinae)

The parasite fauna of many Australian rodents is poorly known. The ectoparasite and helminth faunas of Pseudomys delicatulus, Pseudomys desertor, Pseudomys gracilicaudatus, and Pseudomys hermannsburgensis were determined and compared. In total, 12 species of arthropods, 2 cestodes, and 13 nematodes were found. Species richness of parasites was highest in P. hermannsburgensis and lowest in P. desertor. Despite the sampling effort, the number of parasite species discovered did not reach an asymptote for any of the host species, indicating that the full parasite fauna was not examined. Helminth species richness was highest in the insectivorous P. hermannsburgensis and lower in the obligate herbivores. The structure of parasite component communities was influenced by the social structure of the host species, not surprisingly, with the most highly social species having the highest richness of parasites. Habitat preferences also provided contrast between the helminth component communities, with heligmonellid nematodes occurring in damp woodlands and dominating the parasite fauna of P. gracilicaudatus. Oxyurid nematodes dominated the component communities of the 3 other species, all of which inhabit drier habitats.     

Patterns of infracommunity species richness in eels, Anguilla anguilla

Between October 1999 and October 2001, a total of 510 European eels Anguilla anguilla were captured in 13 different samples from the rivers Thames (five locations) and Test (one location) in southern England. The relationship between parasite component community species richness (CCR) and maximum infracommunity species richness (ICRmax) compared with that previously observed in bird and mammal hosts. Specifically, the maximum number of parasite species occurring in infracommunities equalled or exceeded half the number of parasite species in the component community at that time, across a wide range of CCR values (2-9 parasite species). Furthermore, the frequency distribution of infracommunity richness (ICR) suggested that the species composition of infracommunities is probably random. These findings suggest that intestinal macroparasite infracommunities in eels are unsaturated and potentially species rich assemblages and, in these respects, share a fundamental similarity with the infracommunities of birds and mammals.     

Conservation planning and viability: problems associated with identifying priority sites in Swaziland using species list data

Conservation planning assessments based on species atlas data are known to select planning units containing ecotones because these areas are relatively species‐rich. However, this richness is often dependent on the presence of adjoining core habitat, so populations within these ecotones might not be viable. This suggests that atlas data may also fail to distinguish between planning units that are highly transformed by agriculture or urbanization with those from neighbouring untransformed units. These highly transformed units could also be identified as priority sites, based solely on the presence of species that require adjoining habitat patches to persist. This potential problem was investigated using bird and mammal atlas data from Swaziland and a landcover map and found that: (i) there was no correlation between planning unit species richness and proportion of natural landcover for both taxa; (ii) the priority areas that were identified for both birds and mammals were no less transformed than if the units had been chosen at random and (iii) an approach that aimed to meet conservation targets and minimize transformation levels failed to identify more viable priority areas. This third result probably arose because 4.8% of the bird species and 22% of the mammal species were recorded in only one planning unit, reducing the opportunity to choose between units when aiming to represent each species. Therefore, it is suggested that using species lists to design protected area networks at a fine spatial scale may not conserve species effectively unless population viability data are explicitly included in the analysis.     

The role of helminths in the biological control of mammals

Biological control of invertebrates has been successful while that of vertebrates has been, with the exception of myxomatosis in rabbits, unsuccessful; reasons for this are discussed. Demographic studies of small mammals suggest that population regulation occurs by several different mechanisms, more than one of which may be acting at the same time. Coevolution is an important phenomenon in host-parasite associations, nevertheless parasites may limit host population abundance. The basis of the regulatory effect on the host population is that parasite-induced host mortality or reduction in fecundity is density-dependent. Increasing evidence of the density-dependent effects of helminths on host survival and reproduction is forthcoming from laboratory studies but has not been confirmed in the field. The theory that a helminth parasite may regulate mammal population abundance has been verified recently in the laboratory. A multidisciplinary research programme aimed at understanding the mechanisms responsible for formation of house mouse (Mus domesticus) plagues and seeking strategies to reduce mouse numbers is discussed. One aspect of the work involves investigation of the potential of the nematode, Capillaria hepatica, as a biological agent in the control of wild mice in the cereal-growing regions of Australia. Biological control of mammals is viewed within the context of integrated pest management. A helminth species which reduces host survival or fecundity at an increasing rate as host abundance increases has a role in host population regulation. There is potential to capitalize on that role and apply the helminth as a biological agent in the control of mammals which have attained pest status.     

Host traits and parasite species richness in even and odd-toed hoofed mammals, Artiodactyla and Perissodactyla

Host social, ecological and life history traits are predicted to influence both parasite establishment within host species and the distribution of parasites among host species. Yet only a few studies have investigated the role multiple host traits play in determining patterns of infection across diverse parasite groups. To explore the association between host traits and parasite species richness (PSR), we assembled a comprehensive database encompassing 601 parasites (including viruses, bacteria, protozoa, helminths and arthropods) reported to infect 96 species from two well-studied and diverse host clades: even- and odd-toed hoofed mammals (Artiodactyla and Perissodactyla). Comparative analyses were used to examine associations between three sets of host variables (life history and body mass, social and mating behavior, and ecological traits) and PSR for all parasites combined and for distinct parasite sub-groups. Results from a combination of phylogenetic and non-phylogenetic tests showed that PSR increased with host body size across all parasites groups. Counter to expectations, measures of parasite diversity decreased with host longevity and social group size, and associations between group size and PSR further depended on the underlying mating system of the host species. Our results suggest that body mass, longevity, and social organization influence the diversity and types of parasites reported to infect wild populations of hoofed mammals, and that multiple host and parasite traits can combine in unexpected ways to shape observed patterns.     

中国不同地理区域鸟兽物种丰富度的相关性

生物类群之间物种丰富度的相关性研究是当前物种多样性研究中的热点问题之一,目前,中国尚无相关的研究报道。我们收集了中国三种区域类型：动物地理亚区、行政区和保护区的鸟兽名录,分析了行政区与保护区、动物地理区和经纬度带中鸟兽物种数比值及其相关性。 结果表明：不同区域、动物地理区和经纬度带中鸟兽物种数都显著相关。保护区尺度鸟兽物种 数的相关系数为0.818和动物地理区中的华北区为0.768,其他所有区域和地理区域的鸟兽物 种数的相关系数都高于0.850。因此,鸟兽物种数的相关关系在一定程度上具有预测价值。我们发现不同区域鸟兽物种数比值无显著性差异;但是,不同区域间鸟兽物种数 比值差异显著。该比值在中国呈中间低四周高的分布趋势,其中东北地区最高。我们还利用历史累积调查数据与非历史累积调查数据进行了鸟兽物种数比值及其相关性分析,发现利用累积数据计算的相关性低于非累积数据计算的相关性,但利用累积数据计算的鸟兽物 种数比值高于非累积数据计算的比值。最后,探讨了为什么鸟类与兽类的物种数目会相关。我们根据物种－面积公式,S=CA^Z,导出了两个生物类群物种丰富度的相关关 系式。利用全国不同区域数据拟合,得到Z₁/Z₂＝0.913,Z₁/Z₂接近于1。于是 ,C₁/C2可视为近似等于R_am。本研究可推广到其他不同生物类群物种。物种数量的相关关系为快速评估区域的物种多样性提供了一条途径。